1. Field of the Invention
Aspects of the invention relate generally to wireless communication networks, and more particularly to antennas transferring signals between wireless communication devices and base stations.
2. Description of the Prior Art
Wireless communication networks, such as Personal Communication Service (PCS) networks, provide wireless broadband digital voice, messaging and data services to mobile communication devices like cellular phones. Such a network typically employs multiple base transceiver stations (BTSes) to transmit radio frequency (RF) signals to, and receive signals from, the mobile devices. Often, the BTSes are geographically spaced apart so that each primarily serves a dedicated area, or “cell,” within which each BTS provides communications between mobile devices within the cell and the remainder of the communications network.
Each cell is typically divided into “sectors,” or radial sections of a predetermined angular width. For example, if a cell is apportioned into twelve sectors, each sector is essentially a pie-shaped wedge of thirty degrees in width so that the entire 360-degree circular area about the BTS may be serviced.
In many current PCS implementations, each sector is serviced by a pair of antennas: a first transmit/receive (Tx/Rx) antenna, and a receive-only (Rx-only) antenna. Therefore, two antennas are available to receive signals from wireless communication devices within the cell by way of channels termed upstream links. Similarly, transmission from the BTS to a communication device occurs via downstream links. The receiver antennas are normally separated by at least ten wavelengths of the signals employed in the upstream links to help provide a form of “receiver diversity.” In other words, the two receiver antennas provide slightly different paths, and thus different upstream link characteristics, by which signals from the mobile devices may be received by the BTS. As a result, if the path to one of the antennas is obscured from direct line-of-sight with a mobile device, or if signal reflections from nearby buildings or other objects cause fading of the signal from the mobile device along one path, the other path is likely not affected to the same degree. Thus, this receiver diversity possibly allows communication between the BTS and the mobile device to proceed under a variety of circumstances.
Unfortunately, other problems may result which cannot be resolved by physical separation of the receiver antennas. For example, the Tx/Rx and Rx-only antennas are typically oriented in a fixed vertical position, which is the best orientation with which to receive the normally vertically-polarized signals transmitted from mobile devices, which are typically held in an upright position during use. However, other non-vertical signal polarizations result in a lower-strength signal being received by the antennas. In the worst case, a horizontally-polarized signal oriented perpendicularly to the vertically-oriented antennas will result in a loss in signal strength of 20 decibels (dB) or more as received by the antennas. Such signal loss can result in extreme reduction, or “deep fade,” of the upstream signal, and even termination of current communications between the mobile device and the BTS, often termed a “dropped call.” To counteract such a problem, signal transmission power in the mobile device may be increased, which ultimately results in reduced battery life in the mobile device. Also, reflections, refractions, and other modifications of the upstream signals due to various objects and environmental conditions can alter the polarization of the signals as well, resulting in the communication maladies mentioned above.
To combat problems involving polarization, some receiver antenna systems employ angular diversity, as opposed to the location diversity described above. In one example, each of a pair of linear antennas is tilted at 45 degrees from horizontal so that they cross each other, resulting in a pair of “slant-pole” receiver antennas. This particular configuration mitigates problems regarding a 20 dB loss due to cross-polarization, as no single polarization can be cross-polarized with both slant-pole antennas. However, significant signal strength losses due to polarization mismatches still occur, as such an antenna configuration only exhibits a kind of dual linear polarization.
In other communications systems, some receiver antenna systems employ the use of circular polarization, whereby wireless communication signals may be polarized in a rotating fashion, either mechanically or electronically, about a horizontal axis through all angular orientations. Such antenna systems are commonly employed in satellite communications, and have been proposed in conjunction with citizen's band (CB) radio and PCS communications. However, current circularly polarized receiver antenna systems typically treat all angular orientations equally, and thus do not readily distinguish between wanted communications and potential noise sources.